Ghal-Maraz
Ghal-Maraz is Sigmar's main weapon and, according to some, the "Warhammer" of the franchise, as such it bears special symbolism for all things Warhammer.
Warhammer Fantasy
It was originally a two-handed hammer forged by the dwarfs for the Dwarf king Kurgan Ironbeard, but given to Sigmar as a gift for saving his asses from a mob of Orks. It became the symbol of the Empire and the favorite weapon of the Emperors.
It has some kind of magical power due to the Dwarven rune engraved on it. The said magic helped Sigmar won two important duel. One is where he fought the leader of the Middenland and the hammer was blessed by Ulric after the said leader throw Sigmar into the flame, which Sigmar then shatter the leader's precious long sword along with his skull. The other is where he fought Nagash at the last battle, where the hammer suddenly grows with Dwarven magic from its rune and Sigmar deliver a swift strike at Nagash's rib. The bad chemistry of Dwarven magic mixed with Nagash's dark magic has blown Nagash to pieces.
After Sigmar's absence, the hammer was passed down to Karl Franz where he smashed many orks with the hammer, then to Valten after many people believe he was the reincarnation of Sigmar himself.
During The End Times, Valten carry the hammer to smash many chaos prick while on his way to Middenland. But was ultimately killed by a backstabby Vermin Lord Deceiver after exhausted from his duel with Archaon. The hammer was in Archaon's care for a while until Sigmar manage to reunited with his hammer then smash Ka'bandha's face with it. It allows him to outmatch Archaon in battle but it was not enough to save Mallus (the Warhammer world) with both Sigmar, Ghal-Maraz and Archaon falling through a warp-rift which consumed the planet.
both Sigmar and Ghal-Maraz managed to survive the apocalypse and were merged with the essence of the Realm of Heavens, becoming even more powerful than ever, unfortunately Tzeentch tricked Sigmar into throwing Ghal-Maraz into another warp-rift while trying to strike down Archaon, which ensured Sigmar's withdrawal and the coming of the Age of Chaos.
Age of Sigmar
With the coming of the Age of Sigmar one of the first tasks the God-King gave to the Stormcast Eternals was to recover Ghal-Maraz, they succeded and Sigmar gave the hammer to the Celestial-Prime (we still don't know his true identity but everyone wishes it was Settra the Imperishable or Karl Franz). The Celestial-Prime, now named after the hammer, managed to rescue Alarielle the Everqueen of the Realm of Life from being caught by Nurgle while in the process defeating Chaos Lord Torglug the Despised, killing the chaos guy like the failure he is No wait! You thought it was slay-the-heretic-inquisition-time as usual? Ghal-Maraz is presently imbued with the power of Noblebright, and it purged Torglug, bringing him back as a Stormcast Eternal, that's right kids, Sigmar's main weapon can pull back mortals from corruption (as long as there is still some sliver of soul to be saved, so you better don't try this with Archaon), needless to said, Nurgle needed a lot of ice after that incident.